The invention relates to anaerobic biological treatment of waste streams with high solids content. Anaerobic biological treatment has traditionally been applied to the digestion of primary and secondary sludge at municipal sewage treatment facilities, but is also applicable to municipal solid waste, agricultural manures and crop residues, or industrial solid wastes and slurries where a significant portion of the solids material is potentially biodegradable.
Anaerobic digestion of municipal sludge has been performed for decades to reduce volume, stabilize highly-putrescible material and destroy pathogens. Conventional digestion is a once-through process where the sludge resides in the digester for 20 to 40 days to achieve optimal digestion. This is expressed as solids retention time (SRT) which in a once-through system is equal to the hydraulic retention time (HRT). SRT represents the average time that solids reside in the digester, and HRT represents the average time that liquids reside in the digester. In order to optimize the digestion process and to reduce the size of the digester vessel, there is a need for an improved digestion method that can operate effectively at a reduced SRT.
One problem associated with municipal sludge digestion is the large volume required for the anaerobic digester. Concentrating the solids in municipal sludge upstream of the digester has been used for reducing the digester volume. Even though municipal sludge is relatively high in suspended solids compared to many industrial wastewaters, typically approximately 99% of the municipal sludge may be water. To achieve the conventional SRT, the digester must accommodate the volume of water in the sludge. By concentrating these sludge solids by a factor of two, the digester volume required for digestion could be halved. Traditionally, a thickening process has been applied upstream of the digester to increase the percentage of solids in the feed to the digester. Traditional methods involve mechanical thickeners, dissolved air flotation or similar equipment to concentrate the solids.
Additives, such as polymers, have been mixed with the sludge stream to enhance the thickening process. These polymers are known in the art and include, for example, cationic polyacrylamides in a water-in-oil emulsion, solution mannich polymers—nonionic polyacrylamide polymers made cationic by reacting the amide groups along the polyacrylamide backbone with both a dialkylamine and a formaldehyde source, and cationic water-soluble polymers in emulsions, for example polyamine or poly (diallyldialkylammonium halides). The applicants have discovered that such polymers can inhibit anaerobic biological digestion. This effect might not be noticeable in conventional systems with long SRT and relatively low biological activity. However, as the digestion process is optimized and the SRT is reduced, this impact becomes more noticeable and prevents achievement of optimal digestion performance.
Less conventional methods for thickening the sludge such as membrane separation have also been used upstream of the digester. However, the hydrophilic nature of the solids in the waste stream makes it difficult to extract water efficiently using a membrane separator and promotes fouling of membranes, a build up of colloidal hydrophilic compounds which is difficult to penetrate and disturb. Traditionally, this made membrane separation an unattractive method for thickening the waste stream. Thus, in order to optimize anaerobic sludge digestion, there is a need for an improved method of concentrating the feed stock delivered to the digester and eliminating the need for the above noted polymers in optimized digesters.
Digesters for the anaerobic digestion of municipal publicly-owned treatment works (POTW) sludge are generally large tanks of relatively low height providing for 20-40 days of HRT. Proper treatment of municipal POTW sludge requires a sufficient inventory of active digesting bacteria and contact of those bacteria with the biodegradable fraction of the sludge. Contact is achieved by mixing digester contents. Optimally, the digester contents are mixed thoroughly. Conventional mixing methods include mixing by mechanical methods and mixing by using gas. However, the large and low design of conventional digesters typically results in “dead zones” which are not mixed and which could reach or exceed approximately 15% of the digester.
An “egg-shaped” digester has been developed to address these problems. This shape has improved the overall performance by effectively approaching a 100% mixed digester volume. This digester also requires a smaller widest cross-sectional area because it is taller relative to the traditionally-shaped tanks noted above. However, construction of egg-shaped digesters must overcome complex geometry. Although they are smaller than conventional digesters, they are still relatively large and expensive to construct. These structures improve mixing efficiency, but remain limited by the solids retention time (SRT) dictated by their design parameters and the typical biological activity of a continuously stirred system.
Accordingly, there also remains a need for improved digester performance by exposing more surface area of the degradable organics and available digesting bacteria to increase the opportunity for reactions between them. One way to achieve this is to fragment the sludge particles so as to expose degradable organics and digesting bacteria on the interior of the particles. These components may then be brought into contact in a high-energy environment. This requires turbulent mixing in the digester.
One method of mixing in an anaerobic sludge digester is the loop digester. Loop digesters have a continuous circulating flow which may be around a draft tube configuration. A mixing method used in the field of aerobic digestion is the concept of an eductor nozzle immersed in a liquid filled vessel. The pressure on the pumped side of the nozzle can be used to accelerate the flow of liquid at the nozzle outlet thus releasing energy into the liquid filled vessel and disturbing the vessel contents to effect mixing. Additionally, this acceleration creates a suction effect (similar to a Venturi) which can be used to draw a secondary fluid or gas into the flow stream.
Eductor nozzles to fragment biological solids have been used in the treatment of wastewaters using high rate aerobic digesters that apply a shearing force to the mixed liquor in the digester. With the supplemental addition of oxygen in the form of air injection, these digesters rely on contact between wastewater and biomass particles in an oxygen-rich environment to promote aerobic bacterial digestion of soluble components contained in the wastewater.
Another problem associated with municipal sludge is its disposal. Regulatory restrictions on the disposal of sludge make it desirable that the sludge be treated to “Class A” standards prior to disposal. 40 CFR § 503.32 proscribes EPA standards regarding the use and disposal of sewage sludge and is incorporated herein by reference. Accordingly there is a need for an improved digester design that can provide for operational or process modifications that achieve sludge which is treated to Class A standards.